Electrolytic preparation of sodium ferricyanide



July 18, 1944. H, R NEUMARK y 2,353,782 I ELECTROLYTIC PREPARATION OF SODIUM FERRICYANIDE Filed May 2, 1942 ATTORNEY Patented July 18, 1944 ELECTROLYTIC PREPARATION OF SODIUM FERRICYANIDE Hans R.. Neumark, Forest Hills, N. Y., assignor to General Chemical Company, New York, N. Y., a corporation of New York Application May 2, 1942, Serial No. 441,500

9 Claims.

This invention relates to manufacture of sodium ferricyanide.

It has been proposed to make sodium ferricyanide by electrolytic oxidation of sodium ferrocyanide. Such an operation may be carried out in a cell equipped with an anode and a cathode surrounded by a porous ceramic diaphragm forming the cathode chamber. On passage of current thru the cell, production of sodium ferricyanide is understood to take place as follows:

According to this equation, it will be noted that as oxidation proceeds, for every mol of sodium.

ferricyanide formed one mol of NaOH is also produced. Approximately half of this NaOH is retained in the cathode chamber while the balance of such NaOH Works thru the porous Walls of the diaphragm into the anolyte solution Which, on completion of electrolysis, is withdrawn from the cell.

In an operation of this nature, it is not feasible to carry electrolysis to a point Where substantially all of the sodium ferrocyanide is oxidized to ferricyanide. Hence, a so-called nished sodium ferricyanide anolyte solution contains as an impurity, in addition to a substantial amount of sodium hydroxide, a generally equal quantity of unoxidized sodium errocyanide. To my knowledge substantially pure sodium ferricyanide,

either in liquid or solid form, has not been commercially obtainable. Basis for this diiculty is the very high solubility of sodiumlferricyanide coupled with the relatively high solubility of the sodium hydroxide and sodium ferrocyanide impurities.

The principal object of this invention is provision of a process for making sodium ferricyanide of controllable degree of purity. The invention aims to provide for electrolytic manufacture of sodium ferricyanide by procedure according to which it is possible to remove, substantially completely or to any desired lesser extent, the sodium hydroxide impurity Without introducing extraneous impurities into the sodium ferricyanide liquor and without requiring losses of sodium or cyanide values. A further object lies in the provision of a process by practice of which it is possible to similarly remove, substantially completely or to` any desired smaller degree, the sodium ferrocyanide impurity inherently present in a sodium ferricyanide anolyte formed by commercially feasible electrolytic oxidation of sodium ferrocyanide.

I have found that the objects of the invention, With respect to removal of sodium hydroxide impurity contained in a sodium lferricyanide solution, maybe accomplished by treating the anolyte liquor containing sodium hydroxide with certain agents which function to convert the sodium of the sodium hydroxide to sodium iron-cyanide compounds which are soluble and remainl in solution in the treated liquor, and to convert the hydroxide component of the sodium .hydroxide to an iron hydroxide precipitate which may be removed from the treated solution by simple filtration. Thus, I am enabled to substantially completely neutralize or lower the sodium hydroxide content of anolyte liquorsto any desired extent, to -suit purity requirements of the sodium ferricyanide product, and to transform the sodiumconstituent of the sodium hydroxideto4 a soluble sodium iron-cyanide and remove from the system the hydroxide radical of the sodium hydroxide so neutralized, all Without introducing into the operation any material Which adversely affects purity of the ultimate product, and Without causing loss of sodium or cyanide values by liquor bleed-off as is often necessary in chemical processes.

With regard to removal of lsodium ferrocyanide impurity still present in the -sodium ferricyanide liquor after the desired neutralization ofsodium hydroxide, I iind that there are certainhereinafter described conditions according to which the sodium ferricyanide solutionmay be concentrated in such a Waythat it becomes possible to precipitate-out of the solution, in a readily separable form, substantially all or any desired lesser amount of the sodium ferrocyanide impurity, and

rocyanide to form a sodium ferricyanide liquor -f containing sodium hydroxide and sodium ferroanide impurityy as crystals',,separatingthe latter from the sodium ferricyanide liquor, and'then recovering from `the residual. sodium ferricya-nide solution, substantially pure sodium ferricyanide optionally in liquid or solid form.

In carrying out the invention, in apparatus such as illustrated diagrammatically on the accompanying ow sheet, when starting operation a. sodium ferrocyanide solution may be formed in make-up tank IU, by dissolving in water Na4Fe(CN) 6.101-120 crystals in quantity to form a substantially saturated solution. A sodium ferrocyanide solution as fed into the anolyte cham-Y ber II of an electrolytic cell I2 should contain a certain amount of sodium hydroxide, the function of which is primarily to prevent corrosion of the anode and secondarily to promote conductivity of the anolyte solution. Hence, a desired amount of sodium hydroxide from an external source may be added to the sodium ferrocyanide solution in tank IU, and a typical starting solution may comprise from 200 g. p. l. to 300g. p. l. of Na4Fe(CN)s and from one to 5 g. p. 1. of NaOI-I.

The starting solution is ru-n into the anolyte chamber of the cell provided preferably with a nickel anode and a steel cathode surrounded by a porous ceramic diaphragm I3 forming a cathode chamber I 4. Circulation of solution in the anolyte chamber, and solution temperature of the order of 40-50 C. may be maintained by any convenient means. Ordinarily, during the progress of oxidation, temperature should not exceed-50 C. in order to prevent decomposition of ferricyanide to iron hydroxide, and the temperature maybe kept as high as say 40 C. to avoid crystallization Vof sodium ferricyanide.

' The .cathode chamber is filled with a weak solution of NaOH which functions primarily as a conductor between the cathode and the walls of the surrounding diaphragm. When the current is .turned on', reaction proceeds in accordance with Equation (1), and sodium ferricyanide is formed.` As previously explained, approximate- ,y ly half of the NaOH formed during the course of reaction lWorks its way thru the porous wall of the diaphragm and into the anolyte solution.

' to 350 g. p. l. of Na3Fe(CN) s', from 20 to 30 g. p. l.

of NarFe(CN)s,-and fromf20 to 30 g. p. l. of NaOH.` According to, one example of practice of the process, the anolyte solution at the ,end of electrolysis may analyze 332 g. p. l. of Na3Fe(CN)s, 24 g. p. l. of -Na4Fe(CN)s, and 23 gfp. l. oi NaOH. A feature .of the `present process is directed toward partial or substantially complete removal from the system of the sodium hydroxide which is contained in the finished anolyte. solution and which was produced during oxidation. To this end, on completion of electrolysis, the anolyte solution is run into tank I6 i'or treatment.

, Practice of Vthis purification stepr comprises I treating .a.liquor, of the type described andcontaining sodium lhydroxide with a treating agent reactable with Ysodium hydroxide to vform soluble sodiumV iron-,cyanide and to precipitate iron of the group consisting of ferro and ferri cyanic acids, such salts being reactable with the sodium hydroxide contained in the liquors treated to form soluble sodium iron-cyanide and to precipitate iron hydroxide. Examples of suitable neutralizing or treating agents are Prussian blue, f understood to be ferrie ferrocyanide, Fea(Fe(CN) s) s; Turnbulls blue, understood to be ferrous ferricyanide, Fes(Fe(CN))z; and a more or less white iron salt, understood to be ferrous ierrocyanide, FezFe CN s.

The most satisfactory treating or neutralizing agent is Prussian blue which when added to a sodium ferricyanide solution containing sodium hydroxide reacts with sodium hydroxide in accordance with the following:

(2) 12NaOH-I-Fe4 (Fe (CN) s) 3*) 4Fe (OH) 3+3Na4Fe(CN) s Thus, the sodium of the neutralized sodium hydroxide is converted back to soluble sodium ferrocyanide which is the essential constituent of a starting solution fed into an electrolytic cell and which ferrocyanide may be eventually recycled thru the process by being returned to tank I0. The hydroxide content of the neutralized sodium hydroxide is converted to ferrie hydroxide which may be ltered out of the more or less neutralized liquor as by lter I8.

The amount of treating agent to be used depends upon the purity requirements of the nal sodium ferricyanide product. In some instances, product containing a relatively small amount of sodium hydroxide may be unobjectionable, and in other instances it may be desired to form a product of best feasibly obtainable purity. The NaOH neutralization step of thepresent process is flexible and facilitates 'either substantially complete elimination of NaOH or removal of NaOH to any lesser extent desired. The amount of NaOH present in an anolyte liquor may be, found by analyses and ,the theoretical quantity of treating agent needed to effect the desired degree of puriiication may be readily calculated. In usual practice of the process, I find that an ultimate ferricyanide product of acceptable purity with respect to In the more'` satisfactory embodiments of the invention, it is preferred to control amount of treating agent used so that at the end of the ered to a value not in excess of 8.5.

' theoretical requirements.

treating step, pH of the anolyte solution is 10W- Ordinarily, it is preferred to use in excess of about 5 to 10% by weight of treating agent over the calculated Degree of neutralizationA maybe regulatedby addition to the anolyte liquor being treated o f successive incre- Yments of ltreating agent and by analyses along towardv the'end point of neutralization. I-Iowever, I find that the indicated pH control is the most satisfactory and practicable mode of ad- .iusting neutralization and securing the soughtfor degree of purification. Generally speaking,

in the case of use of Prussian blue as neutralizing agent satisfactory results are obtained when "using about 2.4-2.5 parts by weight for every part of NaOH to be neutralized.

h yclroxide.V VMore particularly I und that'suitableftreating agents are iron salts,o an 'acid Y To further facilitate the neutralizing reaction .in tank I5 and obtain a satisfactory lterable iron hydroxide precipitate, the treating operation should be carried out at temperatures from about 35 C. to not more than 50 C. Since the anolyte liquor discharged from cell I2 is ordinarily within this temperature range preliminary heating of the liquor in tank I is unnecessary although this tank may be provided with a heating coil or other means suitabie to keep the temperature during the neutralizing operation Within the preferred 35 C.-50 C. range. The neutralizing agent is added to the liquor in tank I6 and the mass is agitated from say to 20i minutes during which time the reaction of Equation (2) takes place and proceeds to completion. Thereafter, the liquor is filtered in filter I8 to remove the iron hydroxide which is discharged from the system at I9, the filtrate being run into a vacuum evaporator 2I. A representative neutralized and filtered solution as fed into evaporator 2i may contain from 300 to 350 g. p. l. of Na3Fe(CN) s, fro-m 70 to 90 g. p. l. of Na4le(CN)s, and from 0.1 to 0.5 g. p. l. NaOH. In the particular example of operation of the process, the quantity of Prussian blue (containing 6% H2O) used to effect neutralization of N aOI-I in tank I6 is such as to lower the pH of the anolyte solution to between 8 and 8.5, and is approximately equivalent to 2.4 parts of Prussian blue per part of NaOH neutralized. The neutralized and filtered anolyte as fed into the evaporator 2i may analyze 324 g. p. l. of Na3Fe(CN) s, 78 g. p. l. of Na4Fe(CN) a and 0.3 g. p. l. of NaOH.

When Turnbulls blue or ferrous ferrocyanide are used as neutralizing agents, the reactions taking place are much the same as previously indicated and apparently proceed respectively as follows:

However, it is preferred to use Prussian blue as it has been found that the resulting iron hydroxide precipitate is more readily lterable. The next overall most satisfactory treating agent is Turnbulls blue.

The sodium ferricyanide filtrate of filter I9 contains in solution the sodium ferrocyanide which Was not oxidized during electrolysis plus the sodium ferrocyanide formed in treating tank I6 by the reaction represented by Equation (2). Another feature of the invention consists in the provision of procedure by which it is possible to remove, from the sodium ferricyamde solution, substantially all of such sodium ferrocyanide or of whatever lesser amount needs to be removed to form a ferricyanide product satisfying particular commercial requirements as to ferrocyanide impurity content.

While even in alkaline solution, solubility of sodium ferricyanide is high, I have observed that such solubility decreases appreciably in the presence of increasing amounts of sodium hydroxide, and that the presence of substantial quantity of sodium hydroxide in the sodium ferricyanide solution lowers the solubility of sodium ferricyanide and correspondingly enhances the difculty of making an effective separation of ferricyanide and ferrocyanide. Hence. the neutralizing treatment in tank I6 affords not only removal from the system, as iron hydroxide, of hydroxide component of the sodium hydroxide but also the added important advantage of removal of sodium Vhydroxide from the solution, which correspondingly increases solubility of sodium ferricyanide and facilitates the subsequent separation of sodium ferricyanide and sodium ferrocyanide. Thus, in the following described operation for separation of ferrocyanide from ferricyanide, I ordinarily use a ferricyanide solution which has been neutralized to a pH value not exceeding 10 and preferably not exceeding 8.5.

In accordance with one important feature of the invention, I have found that sodium ferro cyanide contained in the filtrate of filter I8 may be separated from sodium ferricyanide by concentrating the filtrate, at temperature preferably less than that causing appreciable decomposition of ferricyanide, to a ferricyanide content such that on cooling of the concentrated solution to about room temperature, substantially all or a desired lesser amount of the ferrocyanide crystallizes out in crystals of readily separable form. My investigations show that a final sodium ferricyanide product of low ferrocyanide impurity content may be made by concentrating a solution such as a filtrate of filter I8 to a sodium ferricyanide content of not less than 450 g. p. l. To prevent any substantial decomposition of sodium ferricyanide with the resultant formation of sodium ferrocyanide Vand iron hydroxide, the concentrating operation is carried out at temperature not in excess of about 80 C., such temperature being obtained by operating evaporator ZI under sufficiently reduced pressure, e. g., 28-29 inches of Hg. When proceeding under these conditions, there is formed a solution from which, on cooling to around room temperature, sodium ferrocyanide crystallizes out to such an extent that following separation of such ferrocyanide crystals there may be obtained a sodium ferricyanide solution in which the sodium ferrocyanide impurity content ordinarily does not exceed about 0.02%. In the better embodiments of the invention, concentration in evaporator 2| is preferably carried to a sodium ferricyanide content of not less than 500 g. p. l., and where the commercial requirements are such as to call for a sodium ferricyanide product of best feasibly obtainable purity, evaporation is proceeded with to a sodium ferricyanide concentration of 550-575 g. p. l. While any sodium ferricyanide precipitated during this concentration stage Would not constitute a process loss, such sodium ferricyanide would have to be recycled. In order to avoid any appreciable formation of solid ferricyanide before separation of ferrocyanide, it is not desirable to concentrate in evaporator 2! to a ferricyanide content in excess of about 600 g. p. 1.

On completion of concentration, the evaporated liquor isrun into a cooler and crystallizer 23 in which the solution is cooled down to about room temperature, e. g., 15-30" C. Such cooling crystallizes sodium ferrocyanide as These crystals are separated'out in filter 24 and are returned thruline`25 to sodium ferrocyanide i make-up solution in tank I 0. YAccording to this procedure, the sodiumY ferrocyanide which is unoxidized in cell I2 and also the sodium ferrocycontain from 500 to 600 g. p. l. of Na3Fe(CN)s,

0.2 to one g. p. l. of Na4Fe(CN)e, and 0.2 to 0 5 gip. l. of NaOH.` In the illustrative example given, the solution in evaporator 2I 'is concenn trated attemperature not .exceeding 70 C; un-

der vacuum of about 28-29 inches of Hg to an Na3Fe(CN)e concentration of'about 550 g. p. 1., and the concentrated solution cooled to about C. The filtrate of filter 24 may analyze 550 g. p. l. of Na3Fe(CN)s, 0.2 g. p. l. of Na4Fe(CN)s, and 0.3 g. p. l. of NaOH. Although not essential, the sodium ferricyanide solution may be fu; ther ltered again in a clarier 21 to produce in line 28 a crystal-clear solution. Part or all of this sodium ferricyanide solution, drawn off thru line 28, constitutes one of the products of the invention, and may be marketed at the sodium ferricyanide concentration thereof or diluted with water to reducethe sodium ferricyanide concentration, e. g., to a 40% solution of Na3Fe(CN)e.

vIf solid sodium ferricyanide product is desired, part or all of the clear solution is run thru line 29 into a second evaporator 30 in which the solution is further Yconcentrated to a ferricyanide strength such that on cooling to say 15-30 C. a i

good crop of sodium ferricyanide crystals is obtained. Ordinarily, the solution is concentrated in evaporator 30 to a sodium ferricyanide content of about 650 g. p. 1. It is more important to prevent decomposition of sodium ferricyanide in evaporator 30 than in evaporator 2|. While concenerating temperatures in evaporator 30 may be as high as 80 C., preferably concentrating temperatures should not exceed about 65-70 C. The pressure during concentration in evaporator 30 -may be about the same as in evaporator 2|, e. g., 28-29 inches of Hg. On completion of concentration, the solution is run into the cooler and crystallizer, cooled down to about 15-30 C., and crystallized sodium ferricyanide is separated out in filter 33. Wet crystals,

are dried in a drier at preferably maximum temperature of C. The sodium ferricyanide solution filtrate of lter 33 is returned to evaporator 30. According to this Vmode of procedure,

`all of the sodium ferricyanide liquor run into evaporator 30 is eventually recovered as crystal product. VIn the particular example under consideration, in the evaporator 3U, solution containing 550 g. p. l. of NaaFe(CN)e, 0.2 g. p. l. of Na4Fe(CN)e and 0.3 g. p. l..of NaOH is evaporated to ferricyanide concentration of about 630 g. p. 1., and the final product comprises about 99.4% of the Na3Fe(CN)s.2HzO crystals, 0.3%

of NafiFe(CN)s, and 0.01% NaOH.

My copending application Serial No. 441,499, led May 2, 1942, is directedymore particularly to a process for making potassium ferricyanide. r

comprising electrolytically oxidizing a solution of sodium ferrocyanide to form a sodium ferricyanide solution containing sodium hydroxide and sodium ferrocyanide, treating the solution with an iron salt, of an acid of the group consisting of ferroand ferricyanic acids, reactable with sodium hydroxide to form a soluble sodium iron-cyanide and to precipitate iron hydroxide, the quantity of said salt being such as to lower pH of the treated liquor to a value not higher than 10, separating iron hydroxide from the treated solution, concentrating the solution at temperature not in excess of 80 C. to a sodium ferricyanide content of not less than 450 g. p. l., cooling the solution to crystallize sodium ferrocyanide, separating sodium ferrocyanide crystals from the solution, further concentrating the solution, and recovering solid sodium ferricyanide therefrom.

3. The process for making sodium ferricyanide comprising electrolytically oxidizing a solution i of sodium ferrocyanide to form a sodium ferricyanide solution containing sodium hydroxide and sodium ferrocyanide, treating the solution with material of the group consisting of Prussian `blue and Turnbulls blue, reactable with sodium hydroxide to form a soluble sodium iron-cyanide and to precipitate iron hydroxide, the quantity of said material being such as to lower pH of the treated liquor to a value not higher than l0, separating iron hydroxide from the treated solution, concentrating the solution at temperature not in excess of 80 C. to a sodium ferricyanide content of not less than 450 g. p. l., cooling the solution lto crystallize sodium ierrocyanide, and separating sodium errocyanide crystals from the solution.

4. The process for making sodium ferricyanide comprising electrolytically oxidizing a solution of sodium ferrocyanide to form a sodium ierricyanide solution containing sodium hydroxide and `sodium ferrocyanide, treating the solution with Prussian blue, reactable with sodium hydroxide to form sodium ferrocyanide and to precipitate iron hydroxide, the quantity of Prussian blue being such as to lower pH of the treated liquor to a value not higher than 10, separating iron hydroxide from the treated solution, concentrating the solution at temperature not in excess oi 80 VC. to a sodium ferricyanide content of not less l than 450 g. p. l., cooling the solution to crystallize quantity of said salt being such as to lower pH of the treated solution to a value not higher than 10, .separating iron hydroxide from the treated solution, concentrating the solution at temperature not in excess lof 80 C. to a sodium ferricyanide content of not less than 450 g. p. l., cooling the solution to crystallize sodium ferrocyanide, and separating sodium ferrocyanide crystals from the solution. Y Y

2. The process for making sodium ierricyanide sodium ierrOCyanide, separating sodium ferrocyanide crystals from the solution, further concentrating the solution, and recovering solid ferricyanide therefrom.

5. In the process for making sodium ferricyanide by procedure involving electrolytic oxidation of a solution of sodium ferrocyanide to form a sodium ferricyanide solution containing sodium hydroxide and sodium ierrocyanide, and at least partial neutralization of sodium hydroxide, the steps comprising concentrating the solution, having pH value not higher than 10, at temperature not in excess of C. to a sodium ferricyanide content of not less than 450 g. p. l., cooling the solution to crystallize sodium ferrocyanide, and separating sodium ferrocyanide crystals from the solution.

6. The process for making sodium ferricyanide comprising electrolytically oxidizing a solution of sodium ferrocyanide to form a sodium tionwith an iron salt, of an acid of the group consisting of ferroand ferricyanic acids, reactable with sodium hydroxide to form a soluble sodium iron-cyanide and to precipitate iron hydroxide, the quantity of said salt being such as to lower pH of the treated solution to a value not higher than 8.5, separating iron hydroxide from the treated solution, concentrating the solution at temperature not in excess of 80 C. to a sodium ferricyanide content of not less than 500 g. p. l., cooling the solution to about room ternperature to promote crystallization of sodium ferrocyanide, and separating sodium ferrocyanide crystals from the solution.

'7. The process for making sodium ferricyanide comprising electrolytically oxidizing a solution of sodium ferrocyanide to form a sodium ferricyanide solution containing sodium hydroxide and sodium ferrocyanide, treating the solution with an iron salt, of an acid of the group consisting of ferroand erricyanic acids, reactable with sodium hydroxide to form a soluble sodium iron-cyanide and to precipitate iron hydroxide, the quantity of said salt being such as to lower pH of the treated solution to a value not higher than 8.5, separating iron hydroxide from the treated solution, concentrating the solution at temperature not in excess of 80 C. to a sodium ierricyanide content of not less than 500 g. p. l., cooling the solution to about room temperature to promote crystallization of sodium ferrocyanide, separating sodium ferrocyanide crystals` from the solution, further concentrating the solution, and recovering solid sodium ferricyanide therefrom.

8. The process for making sodium ferricyanide comprising electrolytically oxidizing a solution of sodium ferrocyanide to form a sodium ferricyanide solution containing sodium hydroxide and sodium ferrocyanide, treating the solution with material of the group consisting of Prussian blue and Turnbulls blue, reactable with sodium hydroxide to form a soluble sodium ironcyanide and to precipitate iron hydroxide, the quantity of said material being such as to lower pH of the treated solution to a value not higher than 8.5, separating iron hydroxide from the treated solution, concentrating the solution at temperature not in excess of 80 C. to a sodium ferricyanide content of not less than 500 g. p. l. and not more than 600 g. p. l., cooling the solution to about room temperature to promote crystallization of sodium ferrocyanide, and separating sodium ferrocyanide crystals from the solution.

9. The process for making sodium ferricyanide comprising electrolytically oxidizing a solution of sodium ferrocyanide to form a Sodium ferricyanide solution containing sodium hydroxide and sodium ferrocyanide, treating the solution with Prussian blue, reactable with sodium hydroxiole to form sodium ferrocyanide and to precipitate iron hydroxide, the quantity of Prussian blue being such as to lower pH of the treated solution to a value not higher than 8.5, separating iron hydroxide from the treated solution, concentrating the solution at temperature not in excess of 80 C. to a sodium ferricyanide content of not less than 500 g. p. l. and not more than 500 g. p. l., cooling the solution to about room temperature to promote crystallization of sodium ferrocyanide, separating sodium ferrocyanide crystals from the solution, further concentrating the solution, and recovering solid sodium ferricyanide therefrom.

HANS R. NEUMARK. 

